Magnetic field generating system



June 9, 1959 Q wm 2,890,382

MAGNETIC FIELD GENERATING SYSTEM "Filed Nov. 15, 19 56 V s Sheets-Sheet 1 I FIG)! I INVENTOR H John ZewisEdwLn Bale/ (yin June 1959 J. L. E. BALDWIN 2,890,382

MAGNETIC FIELD GENERATING SYSTEM Filed m 15, 1956 3 Sheets-Sheet 2.

' IIVVENTOR John Lewis Edwin Baldwin Filed Nov. 15,- 1956 June 9, 1959 J- L; E. BALDWIN 2,890,382

MAGNETIC FIELD GENERATING SYSTEM 3 Sheets-Sheet 3 FIG 4 4 FIGS FIG.6

INVENTOR F| G.7 John LewL'sEclwLnBaMwLn United States Patent C MAGNETIC FIELD GENERATING SYSTEM John Lewis Edwin Baldwin, Penge, London, England, as-

signor to Rank Cintel Limited, London, England, a British company Application November 15, 1956, Serial No. 622,478

Claims priority, application Great Britain November 18, 1955 6 Claims. (Cl. 315-27) This invention relates to circuit arrangements for producing varying magnetic fields, such as are used for example to deflect the electron beam of a cathode ray tube, and is especially concerned to provide a circuit arrangement such that the magnetic field produced shall be accurately proportional to the integral of an applied controlling voltage.

A known arrangement for producing a magnetic field of which the magnitude varies as a function of an applied controlling voltage comprises an amplifier, usually a thermionic valve having as load a coil system for producing the magnetic field. Signals applied to the amplifier are integrated in the inductive load to provide the desired temporal variation of'the magnetic field. Such an arrangement is satisfactory only if the resistance of the coil system is negligible compared with its inductance, but this is not usually true in practice. It therefore becomes necessary to introduce a correction for the resistance of the coil system and a wide varietyof proposals for this purpose are known. The present invention seeks to provide a particularly advantageous arrangement for introducing this correction.

It'is an object of the vention, therefore,- to provide apparatus for producing a magnetic field of which the magnitude is proportional to the integral of a controlling voltage in which compensation is made for the resistance of the coil system used for generating said field.

It is a subsidiary object of the invention to provide apparatus for producing a magnetic field in which a component electrically included in a feedback network physically associated with a coil system may be located at a distance from said coil system.

" It is a further object of the invention to provide apparatus for producing a magnetic field in which current is fed from a voltage controlled current generator to a coil system by way of a transformer in which compensation is made for losses arising in the transformer.

lt is another object of the invention to provide animproved scan generator apparatus suitable for employment in a television system. 1 .An additional object'of the invention is to provide apparatus for generating a focus correcting field for controlling an electron beam tube. I

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. ,The organization and manner of operation of. the invention may best be understood by reference to thefollovwng description taken in conjunction with the accomanyingdrawings in which:

Figure 1 is a block schematic diagram of one form of apparatus :for carrying out the present invention,

' Figure-2 is a circuit diagram of a preferredembodiment of apparatus according to the present invention,

Figure-3 comprises a series of waveform diagrams illustrativeof the operation of the circuit described in relation to Figure 2, J t 1 Figure 4 shows. an equivalent circuit of'a transformer,

'the amplifier gain is infinite. in practice an appropriate reduction in the time-constant ice 'be either a sum or difference circuit as will be discussed below, to a voltage-controlled generator 3. The output current from this generator is fed to a coil system comprising two coils 4, 5 arranged to produce a magnetic field in a desired space. Across the coils is connected a circuit consisting of a capacitor 6 and resistor 7 in series. In accordance with the present invention the voltage appearing across resistor 7 is fed back to the combining circuit 2 where it is combined with the controlling voltages receivedat terminal 1 'in such a manner as to compensate for the effective resistance of the coil system 4, 5, that is, so that the feedback voltage derived across resistor 7 will be subtracted from the voltage received at terminal 1 to provide the desired compensation.

Where the controlling voltage and the feedback voltage which appears across resistor 7 are combined in a difference circuit the connections ofthe time-constant circuit to the amplifier must be chosen to produce a voltage of the same polarity as the controlling voltage, whereas when the two signals are added, the feedback voltage must be of opposite polarity to the controlling voltage.

The form of feedback voltage derived from the timeconstant circuitwhen the time-constant is made exactly equal to that of the coil system is appropriate only where Since this is not the case of the correcting circuit is made, which in a practical case will often amount to 10%. In other applications, for example, where apparatus according to the invention is used a) develop afocus-correcting magnetic field used to control the electron beam of an electron discharge tube, some further alterations of the norminal time-constant may be found desirable. This reduction is most conveniently' done by reducing the ohmic value of the resistor in the correcting circuit. The correctvalue for this resistor is readily determined by employing a variable resistor which is adjusted until a linear deflection of an electron beam passing through the generated magnetic field is obtained and then replacing the variable resistor by a fixed component of ohmic value equal to that thus found. Whatever the application of apparatus incorporating the present invention, the controlling signal voltage applied to the amplifier means must be the time derivative of the magnetic field which it is sought to generate.

It is advantageous in carrying out the invention to the equivalent series resistance of coils 4, 5. The impedance of the parallel combination of capacitor 6 and resistor 7 withthe coils will then present an impedance which is a close approximation to a frequency-independent resistance, so that the system may be used as the terminating resistance for a transmission line through which it is fed from the generator 3.

A further arrangement which may in some circumstances be found advantageous is to connect resistor 7 to the network of which it forms a part by way of a transmission line having a characteristic impedance of which the modulus has the same value as the ohmic value of the resistor. This arrangement permits the physical separation of the resistor from the network without alteration of its value. The resistor may then conveniently be placed within the combining circuit 2 and connected to coil system 4, 5, by the transmission line. When this is done it may befound advantageous to divide capacitor 6 into two portions, one on.either side .of resistor 7 so thatthe transmission line may be completely isolated from the coil system as regards direct voltages.

In combining the feedback voltage with the controlling voltage it is often convenient to arrange that the voltage appearing across said resistor is applied to one input terminalof a difference amplifier, theothcr input terminal of which has applied to it the controlling voltage which, after amplification with consequent polarity reversal,.is to be integrated to determine the magnitude of the generated magnetic field and the output of which is applied to control the said output stage.

Figure 2 illustrates a preferred embodiment of a circuit arrangement for carrying out the present invention. The elements of this circuit-corresponding to those of Figure 1 are similarlynumbered. In this embodiment the coil system 4, is fed with current from a compensated cathode follower valve 8 which has as cathode load a further valve 9. The .coilsystem 4, 5 is fed from the cathode of valve 8 by way of an isolating capacitor 10. Signals appearing at the anode of valve 8 byreason of a resistor 11 included in the anode circuit are fed by way of capacitor 12 to the grid of valve 9, which-is returned to the negative h.t. line through a resistor 13. The appropriate bias conditions for valve '9 are ensured by a bias resistor 14 in its cathode lead.

Since one terminal of the coil system is earthed, the correcting voltages developed in resistor 7 appear at the junction of resistor -7 with capacitor 6. These potentials are fed by way of a capacitor 15 to the grid of one valve 16 of a difference circuit comprising also a similar valve 17 to which valve 16 is coupled by a common cathode resistor 18. The grid of valve 16 is returned through a resistor-19 to a bias source V.

The controlling voltages applied to terminal 1 are applied to the grid of valve 17 by-Way of a capacitor 20 and the grid is returned to bias source V through a resistor 21. Signalsproportionalto the difference between the voltages applied to the grids of valves 16 and.17 are generated in theanode load resistor 22 of valve 16 and are applied by way of a capacitor 23 to the grid of valve 8, which is returned to a bias source W through a resistor 24.

Figure 3 shows at A, B and C the waveforms of the voltages arising at the grids of valves 17, 16 and 8 respectively when this circuitarrangement is used to generate a magnetic field varying cyclically in a sawtooth fashion. It will be seen that the feedback voltage (B) developed across resistor 7 is of a form which, when subtracted from the input pulse signal (A) gives a combination of pulse and sawtooth voltage waveform (C) which when applied to the resistive inductance of the coil system, 4, 5 is integrated to generate a linearly rising current and hence a linearly increasing. magnetic field.

In a particular embodiment of this circuit which was used togenerate a scanning field for a cathode ray tube repetitive at approximately kc./s. the following component and circuit values were found suitable:

Coils 4, 5-inductance 5 mh., D.C. resistance 89; Capacitor 60.005 ,uf.; resistor 768 kilohms; Valve 8triode-connected 12El; valve 9-l2E1; Capacitor 104,uf.; resistor 11--330t2; Capacitor 12-0.02 f; resistor 13220 kilohms; Resistor 14-270 ohms; capacitor 15-0.02 ,uf.; Valves 16 and 17--EF 91; resistor 1815 kilohms; Resistor 19l megohm; capacitor -200.02 ,uf.; Resistor 21--l megohm; resistor 22-22 kilohms; Capacitor 230.02 Mf.;- 1'SiStO1' Zk-ZZO kilohms; Positive h.t. supply X-Y, +300 volts; negative h.t. supply Y--Z, 300 volts; bias supply'V--Y, 100 v.; bias supply W-Y, --60 v.

The application of the invention to a circuit in which the coil system is fed through a coupling transformer will now be described with reference to Figures 4-7.

Figure 4 shows a circuit which may be considered as the equivalent circuit of a practical transformer when stray capacities are negligible. The circuit components have the following values:

Rp=resistance of primary winding "R' =resistance of secondary winding referred to primary I =leakage inductance due to primary winding Z =leakage inductance due to secondary winding referred to primary L =primary inductance R =iron loss equivalent resistance Such a transformer feeding a deflecting coil system may be represented by the equivalent circuit shown in Figure 5, where the additional components have the following values:

L' =inductance of deflector coils'referred to transformer primary R' =resistance of deflector coils referred to transformer primary.

Consider now the network shown connected'across the transformer primary winding in Figure 6. It may be shown that if the components of this circuit are given appropriate values, the potential arising across -R4 will be identical with that appearing across the pure inductance L of the defiector'coils. The relations between the circuit values which require to be satisfied if an appropriate feedback potential is to appear across R4 are:

C3 and C4 may of course be replaced by a single capacitor of equivalent value.

If a larger feedback output is required, this may be obtained by adopting the modified circuit shown in Figure 7, -in which C1, C2, R1 and R2, obey the relations set out above and in addition As earlier described, the theoretically correct time constants may be modified in practice to obtain compensation for insufficient loop gain.

I claim:

1. Apparatus for producinga magnetic field of which the magnitude is proportional to'the integral ofa controlling signal voltage including means for generating said controlling signal voltage, voltage controlled amplifier means fed with said controlling signal voltage and providing current to a coil system including inductance and resistance and arranged to produce a magnetic field proportional in magnitude to the current therein; means for compensating for the effect of the resistance of said coil system comprising a circuit network including only capacitance and resistance connected effectively in shunt with said .coil system and having a time-constant at least approximately equal to that of said coil system and means for combining a voltage appearing across a resistive component of said network with said controlling signal voltage in such manner as to tend to generate across said coil system a potential having a polarity opposite to that of the potential resulting from said controlling signal voltage.

2. Apparatus as defined in claim 1 for producing a magnetic field in which said circuit network comprises a series combination of a capacitor and at resistor.

3. Apparatus as defined in claim '1 for producing a magnetic field in which said coil system and said resistor each have a terminal connected to ground :and in which the signal appearinguacross said resistor 'isasubtractively combined with said controlling signal voltage.

4. Apparatus as defined in claim 1 for producing a magnetic field in which said network comprises the series combination of a capacitor and a resistor having a value substantially equal to the equivalent series resistance of said coil system.

5. Apparatus for producing a magnetic field of which the magnitude is proportional to the integral of a controlling signal voltage including means for generating said controlling signal voltage, voltage controlled amplifier means fed with said controlling signal voltage and providing current to a coil system including inductance and resistance and arranged to produce a magnetic field proportional in magnitude to the current therein; including a transformer having its primary winding connected in an output circuit of said amplifier and having said coil system connected in its secondary circuit; means for compensating for the effect of the resistance of said coil system comprising a network of resistors and capacitors, said network consisting of a first combination consisting of a first capacitor in series with a first resistor, a second combination consisting of a second capacitor in shunt with a first resistor, said first and second combinations being connected in series effectively in shunt with said coil system, a third combination consisting of a third resistor in series with a third capacitor and a fourth combination consisting of a fourth capacitor in series with a fourth resistor, said third and fourth combinations being connected in series and shunted across said second combination, the magnitudes of the resistances and capacitances of said network being so chosen that it presents a time-constant substantially equal to the effective timeconstant of said coil system referred to the primary winding of said transformer and means for combining voltages appearing across said fourth resistor with said controlling signal voltage.

6. Apparatus for producing a magnetic field of which the magnitude is proportional to the integral of a controlling signal voltage including means for generating said controlling signal voltage, voltage controlled amplifier means fed with said controlling signal voltage and providing current to a coil system including inductance and resistance and arranged to produce a magnetic field proportional to the current therein including a transformer having its primary winding connected in an output circuit of said amplifier and having said coil system connected in its secondary circuit; means for compensating for the effect of the resistance of said coil system and for losses in said transformer, comprising a network of resistors and capacitors said network consisting of a first combination consisting of a first capacitor in series with a first resistor, a second combination consisting of a second capacitor in shunt with a second resistor, said first and second combinations being connected in series effectively in shunt with said coil system, a third combination consisting of a third resistor in series with a third capacitor and a fourth combination consisting of a fourth capacitor in series with a fourth resistor, said third and fourth combinations being connected in series and shunted across said second combination, the magnitudes of the resistances and capacitances of said network being so chosen that it presents a time-constant substantially equal to the effective time-constant of said coil system and means for combining voltages appearing across said fourth resistor with said controlling signal voltage, the product of the values of said first resistor and capacitor being equal to the product of the value of the leakage inductance of the primary winding of said transformer and the reciprocal of the resistance of the transformer primary winding, the product of the values of said second resistor and capacitor being equal to the product of the primary inductance and the reciprocal of the iron loss equivalent resistance, the product of the values of said third capacitor and resistor being equal to the product of the secondary leakage in ductance and the reciprocal of the resistance of the secondary winding, the product of the values of said fourth capacitor and resistor being equal to the product of the inductance of said coil system referred to the primary winding of said transformer and the reciprocal of the resistance of said coil system referred to the primary winding of said transformer and the quotient of the value of said first resistor divided by the primary leakage inductance being equal to the quotient of the value of said second resistor divided by the primary inductance, to the quotient of the value of said third resistor divided by the secondary leakage inductance and to the quotient of the value of said fourth resistor divided by the resistance of the secondary winding referred to the primary winding.

References Cited in the file of this patent UNITED STATES PATENTS 2,093,177 Vance Sept. 14, 1937 2,574,365 Wynn Nov. 6, 1951 2,713,651 Cofiey July 19, 1955 

